“Sashi,” which is the Japanese translation of “hole,” refers to the muscular liquefaction, or “pus pockets,” in tuna. A similar condition can occur in swordfish as well as other species, and is often referred to as “kudoa.” The basic cause is from infestations of myxosporidian protozoan parasites.
In tuna, the myxosporidian parasite Hexacapsula neothunni colonies form white, spherical, 2-4 mm diameter masses in the freshly-caught tuna. As the fish deteriorates post mortem, the colony begins to release proteolytic enzymes that liquefy the muscle adjacent to the colony.* If the fish is subjected to temperature abuse (not kept cold), the enzymatic process accelerates to where the fish musculature is riddled with puss-filled holes, and disintegrates the structural integrity of the meat. Although this condition does not represent a public health risk, it can be extremely unsightly and is a costly, troublesome reality in the fresh tuna market.
The frequency of these naturally-occurring parsites depend on many factors. Protozoan invaders or their propagative stages are ever present around their potential hosts. Stress conditions and other ecological factors (water temperature, currents, diet, etc.) can overwhelm the protective mechanisms of the wild fish. The same fishing area may have seasons with higher infestation rates than other times of the year. The stage of advancement of the parasitic colony can be an indication of the on-board handling of the tuna and the remaining shelf life. It also makes a very large difference in the price or value of the fish.
In swordfish and other commercial fishes, the specific myxosporidian Kudo musculoliquefaciens is associated with muscular liquefaction, and in mahi-mahi, Kudo cruciformum is suspected of causing “pus pockets” condition to the meat. Similarly, “jelly-meat,” appearance in halibut and other species is suspect to be caused by another class of myxosporidian parasites, causing a jellified, or “milky” appearance to the meat.
Although uncommon, the effects of these naturally-occurring parasites are a reality in the commercialization of many fish species, and should be factored into the cost and operation of normal business.
*Researchers are not certain whether the enzymes that liquefy muscle are from the parasite or from the fish. The current theory is that while the fish is living, it lines up an immune response to surround the parasite and keep it in its place. Once the fish dies, these immune cells also die and release the enzymes (lysozymes) which begin to breakdown the connective tissue and surrounding muscle, which forms the “pus pockets” where the colonies were located.
References: John Kaneko (PacMar, Inc.), Jiri Lam, (Insitute of Parsitology, Prague)